It is very difficult for Group III nitride semiconductors GaN, AlN, InGaN and AlGaN to grow into a large-sized bulk single crystal, and therefore, heteroepitaxial growth using sapphire as the substrate has been generally employed. However, a lattice mismatch of 11 to 23% and a thermal expansion coefficient difference of up to 2×10−6/° C. are present between Group III nitride semiconductors and sapphire. Also, because of difference in chemical properties, a Group III nitride semiconductor epitaxial film directly grown on sapphire only partially inherits the properties as a single crystal of the substrate and grows three-dimensionally, and this is thought to make it significantly difficult to maintain a flat surface profile. It is necessary that the substrate on which a GaN single crystal film is grown have, heat resistance up to 1,200° C. and no reaction with NH3 at that temperature. In this respect, only sapphire and SiC are currently available as a substrate which is producible at practical cost. Of these, sapphire is overwhelmingly advantageous from a cost comparison view-point, and 90% or more of the GaN-based light-emitting device (LED) actually produced in the world employs a sapphire substrate. However, sapphire and GaN differ in the lattice constant and thermal expansion coefficient and further in the chemical properties, and therefore direct growth of a GaN single crystal is believed to be impossible. Consequently, despite various modifications and vast improvements made in a GaN-based light-emitting device formed on a sapphire substrate, a fairly high number of defects occur on the inside, and there is a limit to sufficiently enhancing the luminous efficiency and element lifespan.
Generally, the method for obtaining a single crystal film having good crystallinity by heteroepitaxial growth with a large lattice mismatch includes the following two ways.
(i) Growth through a material having physical constants intermediate between the substrate and the epitaxial film, whereby the quality of the epitaxial film can be enhanced. That is, a thin film having intermediate properties in terms of lattice constant, chemical properties, thermal expansion coefficient and the like is interposed. In this case, a single-crystal thin film must be inserted, because the properties of the single crystal substrate are desired to be inherited by a single crystal as directly as possible.
(ii) Interpose a polycrystalline or amorphous film formed of the same material as that of the objective single crystal thin film. The film above is usually formed by a method of depositing a film at a temperature lower than the single crystal growth temperature (Kokoku (Japanese Examined Patent Publication) No. 62-29397). Epitaxial growth of SOS (silicon on sapphire substrate) or the like was first studied, which results in a low-temperature buffer layer for GaN on a sapphire substrate. The mechanism is based on the fact that GaN yields a high nucleation density on the buffer layer, generation of a grain boundary is inhibited due to selective growth/coalescence of only crystal grains well-aligned in the crystal orientation out of the crystal grains, and flattening is achieved by utilizing a higher rate of growth in the lateral growth direction on the buffer layer (see, for example, Isamu Akazaki, et al., Journal of the Japanese Association for Crystal Growth, Vol. 13, No. 4, 1986, pp. 218-225; Vol. 15, No. 3-4, 1988, pp. 334-342; and Vol. 20, No. 4, 1993, pp. 346-354).
Trend (i) means that the quality of an epitaxial film can be enhanced by performing the growth through a material having physical constants intermediate between the substrate and the epitaxial film. Accordingly, growth via an AlN layer is considered to be effective for growing a GaN layer on a sapphire substrate. Since, the lattice constant and thermal expansion coefficient of AlN are intermediate between sapphire and GaN and therefore, lattice mismatch and thermal distortion are efficiently moderated. In addition, AlN and GaN are close in the chemical properties, and the interfacial energy therebetween is also small. In other words, this may be understood as follows. Sapphire, i.e., Al2O3, is an oxide, and the most chemically similar nitride is AlN having Al in common. The lattice mismatch is 11% and relatively high, but by having Al in common, an AlN single crystal is liable to grow. Moreover, since AlN is the only compound allowing 100 percent solid solution of GaN, the chemical properties are closest to each other and the lattice mismatch is only 2%. Accordingly, even though direct growth of Al2O3/GaN may be difficult, when AlN is inserted as in Al2O3/AlN/GaN, a GaN single crystal can be grown while inheriting the crystallinity of sapphire (Al2O3). So, once a flat AlN layer can be formed directly as a single crystal, the GaN film quality of heteroepitaxial film grown thereon can be remarkably enhanced.
The following three methods are known to deposit an AlN film for this purpose.
I. A method of heat-treating a sapphire substrate in a nitrogen source gas atmosphere such as NH3, N2H2 and organic amine, thereby converting the substrate surface into single crystal AlN (Kokoku No. 7-54806), or a chemical vapor deposition method of vapor-depositing Al in an NH3 or N2H2 atmosphere (Kokoku No. 59-48796).
II. A method of supplying an aluminum source gas such as organic aluminum, halogenated aluminum and metal aluminum vapor, and a nitrogen source gas onto a sapphire substrate kept at a high temperature allowing for single crystal growth of AlN, to deposit a single crystal AlN layer (Kokai (Japanese Unexamined Patent Publication) No. 9-64477), where a high temperature of about 1,300° C. is required.
III. A method of supplying an aluminum source gas and a nitrogen source gas at a low temperature of 500 to 1,000° C. to deposit a polycrystalline or amorphous AlN layer of several hundreds to 1,000 Å, and annealing it at a higher temperature, thereby forming a single crystal (Kokoku No. 4-15200 and Kokai No. 5-41541).
Regarding method I above, in the case of surface nitriding, a nitride layer of several tens of A can be formed with good reproducibility and moreover, formation of this single crystal AlN layer involves a gradient compositional change, so that the lattice mismatch can be effectively moderated to a region of only several tens of Å. In the chemical vapor deposition, an ultrahigh vacuum of 10−8 Torr is required, and Al vapor and NH3 or N2H2 are reacted on a substrate at a high temperature of 1,000 to 1,200° C. However, the AlN layer formed by such a method does not result in a nitriding reaction to uniformly proceed and may have surface roughness on the order of 10 Å. When epitaxial growth is performed on the surface-roughened AlN layer, the irregularities are exaggerated with an increase in the film thickness and a flat surface profile cannot be obtained.
On the other hand, in the AlN layer formed by the method II, uniform and fine growth nuclei cannot be generated simultaneously due to film growth at a high temperature, but sequential generation of nuclei results and therefore, three-dimensional growth is unavoidable. Ito et al. have stated that a GaN crystal with smooth surface cannot be obtained unless the mechanism used in the low-temperature buffer works, i.e., the mechanism where uniform and fine polycrystals are produced simultaneously while inhibiting single crystal growth by reducing the flow rate of NH3 as much as possible even when growing AlN at a high temperature and a smooth surface is created by promoting the lateral growth (J. Crystal Growth, 205 (1999), pp. 20-24).
In this way, the single crystal AlN layer formed by the methods I and II has a certain effect in improving the crystallinity of the epitaxial film grown thereon and enhancing the optical characteristics such as PL (photoluminescence), but three-dimensional growth is promoted to create an irregular surface which makes it difficult to obtain an epiwafer enabling fabrication of an LED device that is reliable even when current flows.
In the method III, an AlN film is deposited at such a low temperature that no three-dimensional growth occurs, so that a flat amorphous layer can be formed. However, annealing the single crystal formation involves a minute difference in the orientation between the first crystallized portion and the later crystallized portion and therefore, the surface begins to become disordered. When a GaN epitaxial film is grown thereon, irregularities are gradually produced.
As described above, the method of using a single crystal AlN seed layer with intermediate physical constants in the heteroepitaxial growth of growing a GaN single crystal on a sapphire substrate has been studied but has stopped at present due to not being able to maintain surface flatness.
Therefore, a buffer layer in trend (ii) instead of (i) above is currently being employed. In use as a buffer layer, it is meaningless to have intermediate physical constants, and the basic practice is to use a microcrystalline or amorphous thin film having the same composition as that of the single crystal intended to grow. For this reason, a low-temperature buffer method using, as the buffer, a layer formed by depositing a GaN film at a low temperature near 500° C. is being most widely employed.
Meanwhile, a sputtering method has long been studied as the method for obtaining an AlN film with a uniform thickness. A. J. Shuskus et al. have reported as follows (Applied Physics Letters, Vol. 24, No. 4 (1974), pp. 155-156). That is, a high-purity Al target was subjected to RF discharge in NH3 gas by using a reaction vessel capable of reaching a vacuum of 10−8 Torr, and an AlN film was deposited on a (0001) plane sapphire substrate at 1,200° C., whereby a single crystal thin film as measured with reflective electron beam analysis could be formed. However, the obtained AlN film exhibits only one kind of pattern in the reflection electron beam diffraction, and they are silent on the absence of columnar crystal grain boundary and the surface properties. After that, C. R. Aita et al. formed an AlN thin film on single crystal Si at room temperature by using a high-purity Al target and discharging a mixed gas of Ar and N2 and closely examined the discharge conditions and the quality of the film deposited (J. Appl. Phys., Vol. 53, No. 3 (1982), pp. 1807-1809, J. Vac. Sci. Technol. A, Vol. 1, No. 2 (1983), pp. 403-406). Also, W. J. Meng et al. performed an experiment in which a film was deposited on Si(111) and Si(100) substrates under the same conditions by raising the temperature to 600° C. or more and have reported that an AlN thin film having a very fine smooth polycrystalline surface whose orientation is aligned with the C-plane was formed on both substrates (J. Appl. Phys., Vol. 75, No. 7 (1994), pp. 3446-3455). Despite subsequent various discussions on its application as a compound semiconductor, this falls short of being practical, because the energy gap of AlN is as large as 6.2 eV.
A high-energy electron flow is produced upon plasma generation and when the electron is implanted into a crystal, a defect called plasma damage is created in the crystal. Therefore, a sputtering method has not been actively used in the semiconductor application where a thin-film crystal reduced in the defect as much as possible is required. However, since a sputtering method is a very excellent method for depositing a thin film of several tens to several hundreds of Å with good reproducibility and becomes widespread through its proven performance in stably mass-producing a highly functional thin film as a thin-film multilayer in the Si semiconductor wiring process or in the field of hard disk media or head, the sputtering method is now aggressively studied. The AlN film formed by the sputtering method is amorphous or polycrystalline in many cases, and there are a very few reports where a single crystal film is deposited. In particular, as implied by the term “plasma damage”, it is generally believed that when a single crystal is exposed to plasma, the crystal is damaged. In this way, the sputtering method is a highly advantageous method as the method of depositing a film while maintaining the substrate flatness but is seldom considered as the method for raising the crystallinity.
On the other hand, the low-temperature buffer approach is based on the fact that uniform and fine polycrystal nuclei generate simultaneously to allow coalescence of only crystals aligned in the orientation, where a flat single crystal can be formed by utilizing the lateral growth. Therefore, a polycrystalline or amorphous thin film needs to be uniformly deposited. In this connection, use of AlN in the sputtering method of film-depositing a low-temperature buffer has emerged as one approach. That is, an amorphous AlN or GaN film is deposited by reaction sputtering using an Al or Ga target and after once removing the film from the apparatus, GaN is grown by MOCVD (Kokai Nos. 2000-286202, 2001-94150 and 60-173829).
In 1972, Cuomo et al. succeeded in depositing a polycrystalline thin film aligned in the GaN orientation by reaction sputtering using a Ga target for a sapphire substrate (Appl. Phys. Lett., Vol. 20, No. 2 (1972), pp. 71-72, and Kokai No. 48-40699), and furthermore, by developing this technique, a method of producing a buffer layer and an underlying layer by sputtering has been proposed (U.S. Pat. Nos. 6,692,568 and 6,784,085, and Kohyo (National Publication of Translated Version) No. 2004-523450), where a large number of columnar crystals are generated on a substrate and a single crystal GaN thin film is obtained on the columnar crystals by utilizing the lateral growth, i.e., by modifying the apparatus or varying the conditions such as ratio of Ar to N2 and discharge power, to allow coalescence of only crystals substantially aligned in the orientation out of the columnar crystals (see, for example, FIG. 4 of U.S. Pat. No. 6,692,568).
A technique of partially creating a portion disallowing growth and promoting the lateral growth, thereby improving the crystallinity, is known as the high crystallization technique for heteroepitaxial growth. Production of a crystal with a low defect density has been attempted by applying this technique to a GaN single crystal thin film, and successful results have been achieved.
I. Before Application to GaN-Based Semiconductor Laser Diode (LD)
As the method for growing a crystal such as GaAs on an Si substrate, the following publications are disclosed before the technique of growing GaN on a sapphire substrate is spread.
Kokai No. 57-115849 is characterized by comprising the following steps (α) to (δ) and includes the selective/lateral growth concept. In the Examples, GaAs is grown on an Si substrate.
(α) A step of performing epitaxial growth on a substrate,
(β) a step of patterning the epitaxial growth layer in a grid, channeled or dot fashion by photoetching,
(γ) a step of again performing epitaxial growth on the substrate, and
(δ) a step of polishing the substrate to flatten the epitaxial layer.
Kokoku No. 06-105797 describes a semiconductor substrate with a compound semiconductor epitaxial growth layer, comprising a compound semiconductor substrate having on the surface thereof an insulator thin film or high melting point metal thin film in which a plurality of window parts are partially provided, wherein the substrate portion exposed in the window part of the thin film is used as a seed and a portion continuously grown from the seed part in the direction parallel to the substrate surface is joined and integrated with an epitaxial growth portion grown from the adjacent seed part, and this technique includes selective growth/lateral growth.
Kokai No. 04-127521 describes a technique of forming an SiO2 mask on a GaAs substrate and growing InGaAs by liquid-phase epitaxial growth.
Kokai No. 04-303920 describes a technique of forming an SiO2 mask on an Si substrate and laterally growing GaAs through growth by MOCVD. It is disclosed that since defects are decreased on the mask but not decreased in the mask-free portion, a mask is again formed when flattening can be achieved, and GaAs is grown thereon.
II. Development of Application to GaN-Based Semiconductor LD
A GaN-based LED having a double heterostructure using a quantum well structure was fabricated in 1996 and since then, leading-edge development was focused on LD. A blue LED whose output at 20 mA exceeds 5 mW was deemed as having ultrahigh luminance at that time, but when the crystallinity was evaluated by the threading dislocation density, the crystal was in a level allowing threading dislocations to exist at a density of about 10+9/cm2 and the dislocation density was four-digit higher than that of the crystal used in GaAs-based LED. In LD fabrication laser oscillation is not easily confirmed. In the case of LD, a larger number of defects leads to a higher threshold current oscillated, and a higher threshold current gives rise to a higher evolution of useless heat. Continuous oscillation for a long time is required in practical use and for this purpose, the threshold current needs to be greatly reduced. Good crystallinity is essential for reducing the threshold current. The theory that luminous efficiency of an LED, does not change even when the crystallinity is increased (Science 14 Aug. 1998, Vol. 281, No. 5379, pp. 951-956) is widely accepted, and the focus of research has shifted to increasing the light collection efficiency, but in order to finalize the fabrication of LD, an increase in crystallinity is necessary, and hence so-called “selective/lateral growth method” is drawing attention as a method therefor.
It was reported by Y. Kato et al. in 1994 that when a part of the growth surface for GaN in MOCVD is masked with SiO2 and so-called “selective growth” occurs, a {1-101} plane facet appears in a window in the <11-20> direction (JCG, 144 (1994), 133-140).
Furthermore, a dot pattern of hexagonal pyramid surrounded by six {1-101} planes was grown on sapphire by them (JJAP, 34 (1995), L1184-L1186). These techniques are not aimed at increasing the crystallinity, but the process is selective/lateral growth and is positioned as the starting point for selective/lateral growth in the GaN-based semiconductor.
In 1997, A. Usui et al. formed a crystal having a threading dislocation density of 6×10+7 cm2 by putting an SiO2 mask on a part of the sapphire/LTGaN buffer/GaN surface and growing GaN of 30 μm or more in thickness by the HPVE method. The crystal was taken out during growth and observed with SEM, and the process of a facet being created in the early time of growth and gradually filled was thereby confirmed (JJAP, 36 (1997), L899-L902). Subsequently, it was confirmed by TEM observation that in the course of filling the facet after its creation, the threading dislocation is bent to the direction parallel to the substrate surface and the number of dislocations is decreased when filling of the facet is finished (APL, 71 (1997), 2259-2261; APL, 73 (1998), 481-483).
R. Davis et al. caused lateral growth to proceed by putting an SiO2 mask on SiC substrate/AlN buffer/GaN and forming a striped window and in 1997, reported that when the cross-section was observed by TEM, only a few dislocations were present on SiO2 but numerous dislocations were observed in a portion without SiO2. In this case, the growing process utterly differs between the window direction being <11-20> direction and being <1-100> direction, and in the <11-20> direction, a {1-101} plane facet is finished to give a triangular cross-section, but in the <1-100> direction, the cross-section shows a rectangular profile defined by the (0001) plane as the top surface and the {11-20} plane as the side surface (APL, 71 (1997), 2638). Also, it is indicated that as the gas pressure during growth is higher, as the area ratio of the mask is smaller, and as the TEG concentration is higher, the {1-101} plane facet is more readily created (JJAP, 36 (1997), L532). In 1999, the group lead by the professor R. Davis above proposed a selective/lateral growth method called a PENDEO method (J. NSR, 4S1, (1999), G3, No. 38, and APL, 75 (1999), 196). This is an approach of mainly growing the {11-20} plane. After preparing SiC/AlN buffer/GaN, a film obtained by stacking Ni is used as the mask, the portion with an window open is dug to SiC by etching, and Ni is then removed to grow GaN. GaN is not grown on SiC, and the {11-20} plane of GaN exposed by etching mainly grows. According to the cross-sectional SEM, a space is opened in the portion where AlN buffer/GaN are not present. Laterally grown GaN collides and covers the entire surface and thereafter, the (0001) plane grows. The crystal grows in the space until it collides.
At almost the same time, S. Nakamura et al. increased the performance of LD by applying selective/lateral growth to LD (Appl. Phys. Lett., Vol. 72 (1998), 211, and Jpn. J. Appl. Phys., Vol. 36 (1997), L1568), where the {11-20} plane was grown by forming an SiO2 mask on sapphire/LT GaN buffer/2-μm GaN with an window open in the <1-100> direction. According to this method, the number of dislocations is small on SiO2 but is large in the portion with the window open and therefore, a technique of fabricating an element by selecting a portion having little dislocations is employed. The cleavage plane differs between sapphire and GaN and if the direction having little defects and allowing for fabrication of an element is decided, a plane for laser oscillation cannot be created by cleavage. Therefore, GaN was stacked to a large thickness of 100 μm, and sapphire was shaved off by polishing. That is, a GaN single crystal substrate was formed by selective/lateral growth and an LD structure was grown thereon (Jpn. J. Appl. Phys., Vol. 37 (1998), L309, and Appl. Phys. Lett., Vol. 72 (1998), 2014). As a result, continuous oscillation for 10,000 hours became feasible at room temperature, and sales of this element were announced in 1999. Triggered by this success, it is acknowledged that selective/lateral growth is essential for fabricating ultraviolet/blue LD.
III. Classification of Selective/Lateral Growth Methods
The selective/lateral growth is roughly classified by the portion used for growth and the method for selective growth, into growth aiming at preparation of a GaN single crystal substrate by forming a thick film and removing sapphire, and growth of forming a thin film to raise the crystallinity of the underlying layer.
In applying selective/lateral growth, the method for raising the crystallinity of the underlying layer can be classified by the step of selecting the growth portion, the method for selecting the portion, and the kind of the plane that is preferentially grown when performing lateral growth.
A Step of Selecting the Growth Portion:
(α) the portion is selected in the sapphire substrate itself,
(β) the portion is selected in the buffer layer, or
(γ) after GaN is once grown, the portion is selected in the GaN surface.
B The Method for Selecting the Portion Includes the Following Three Kinds of Methods:
(a) covering with a mask, and
(b) removal by etching or the like of the plane where a single crystal cannot grow.
C The plane that is preferentially grown when performing lateral growth includes the following two kinds of planes:
(i) a facet oblique with respect to the substrate, such as {1-101} plane, is used; or
(ii) a facet in the direction perpendicular to the substrate, such as {11-20} plane, is used, that is, the crystal is grown in the direction parallel to the substrate.
However, in conventional techniques, crystallinity of the crystal as a basis for selective/lateral growth is not so good, leading to failure in obtaining sufficiently high crystallinity by one selective/lateral growth, and the process is repeated twice or the level usable for LD is not reached.
As described above, the method for heteroepitaxially growing a GaN-based semiconductor on a sapphire substrate includes two ways of thinking, i.e., (i) a method of interposing a single crystal seed layer having intermediate physical and chemical properties, and (ii) a method of forming a buffer layer for simultaneously generating uniform and fine nuclei of a polycrystalline or amorphous material with the same composition as that of the objective single crystal and allowing coalescence/growth of only crystals aligned in orientation, and the method (ii) is spread. A sputtering method has been considered and widely studied as the method for forming a thin film while maintaining flatness of the sapphire substrate. This film is effective as a polycrystalline or amorphous buffer layer, but its use as a flat single crystal seed film has not been studied. Because, the sputtering is generally thought to be unsuitable as the method for forming a single crystal.
As described above, with respect to the method of inserting a single crystal thin layer in keeping with the approach (i), three-dimensional growth cannot be prevented in the conventional methods, and even when the surface roughness of the sapphire substrate is about Ra=0.8 Å, Ra of the thin film formed thereon becomes 10 Å or more. Use of a low-temperature buffer layer allows partial formation of a columnar crystal when the temperature is raised to deposit a GaN-based semiconductor film after film deposition of the buffer layer and therefore, the surface flatness in terms of Ra also becomes 10 Å or more.
In the present invention, unlike the low-temperature buffer layer of the currently predominating trend (ii), a GaN-based crystal is obtained in line with the approach (i) that is little studied at present. The failure in most conventional methods in (i) is ascribable to the fact that surface flatness is largely roughened when an AlN thin film is deposited, compared with the sapphire wafer surface.
As described above, GaN does not grow directly on a sapphire crystal and therefore, an AlN or GaN buffer layer was inserted to reduce the crystal mismatch, whereby a success in growing a remarkably excellent GaN crystal for that time was achieved and the luminescence intensity of LED was enhanced to a practically durable level. As a result, LED using a GaN-based crystal was employed as the backlight of a liquid crystal display for cellular phones and this triggered expansion of the demand therefor at a rate of more than 50% per year. In recent years, studies are proceeding toward using an LED backlight as a backlight for personal computer monitors and TVs which are the same in terms of a liquid crystal display. Then, it has been found that sufficient luminous efficiency and reliability cannot be obtained with conventional crystallinity, and demands for higher crystallinity are increasing. The heteroepitaxial growth includes the following two methods. The first is a method of inserting a single crystal seed layer having intermediate physical and chemical properties, and the second is a method of using a buffer layer allowing for simultaneous generation of uniform and fine polycrystalline or amorphous nuclei of a material having the same composition as that of the single crystal and lateral coalescence of crystals aligned in the orientation. Of these, the method using a low-temperature buffer is currently predominating for GaN-based semiconductors. However, as long as a buffer layer is inserted, the regularly ordered atomic arrangement of the single crystal of the substrate is once broken, or portions differing in the crystallization level are generated due to partial progress of crystallization in the course of heating the low-temperature buffer to the growth temperature, and the flatness of the surface is impaired. Accordingly, high crystallinity required at present is considered to be extremely difficult to achieve.